Isocyanates containing alkoxysilane groups are usable in a versatile manner as heterofunctional units and may find use, for example, in coatings, sealants, adhesives and elastomer materials, but are not limited to these fields of use.
Processes for preparing isocyanates containing alkoxysilane groups are known. For example, they can be obtained by reacting alkoxysilanoalkylamines with phosgene in the presence of tertiary amines (DE 35 44 601 C2, U.S. Pat. No. 9,309,271 B2), although not only the toxicity of phosgene but also the formation of chlorinated by-products and salts is disadvantageous. Alternatively, access to isocyanates containing alkoxysilane groups can also be achieved via hydrosilylation of isocyanates containing olefin groups in the presence of precious metal catalysts (EP 0 709 392 B1). Disadvantages here are generally inadequate selectivity and high catalyst demand.
A further route to alkoxysilane-containing isocyanates leads via the reaction of haloalkylalkoxysilanes with metal cyanates to form alkoxysilanoalkylurethanes and subsequent thermal cleavage of the urethanes to release the corresponding isocyanates (U.S. Pat. Nos. 3,821,218 A, 3,598,852 A, DE 35 24 2015 A1). Disadvantages here are the formation of large amounts of salt and the need to use a solvent, which is typically dimethylformamide.
U.S. Pat. No. 5,218,133 A describes a route to preparation of alkoxysilanoalkylurethanes that avoids the troublesome formation of stoichiometric amounts of salt. For this purpose, alkoxysilanoalkylamines are reacted with alkyl carbonates in the presence of basic catalysts, especially in the presence of metal alkoxides, and the reaction mixture is then neutralized.
U.S. Pat. No. 5,393,910 A describes a process for thermal cracking of alkoxysilanoalkylurethanes prepared preferably according to U.S. Pat. No. 5,218,133 A at high temperature in the gas phase. A disadvantage of this process is the need for special equipment which is stable to high temperature and thus costly. Moreover, patents that do not relate specifically to silanoisocyanates report that the high temperature required leads to reactor carbonization. This is disadvantageous because it is detrimental to plant availability.
As an alternative to urethane cleavage in the gas phase, the thermally induced release of isocyanates containing alkoxysilane groups can also be conducted in a dilute manner in inert solvents (see U.S. Pat. Nos. 5,886,205 A, 6,008,396 A). This involves adding the alkoxysilanoalkylurethane to the inert solvent and choosing a sufficiently high temperature for the solvent as to promote urethane cleavage on the one hand but to avoid unwanted side reactions as far as possible on the other hand. U.S. Pat. No. 5,886,205 A discloses, for the reaction performable in a batchwise or continuous manner, pH values of less than 8, temperatures of not more than 350° C. and a catalyst comprising at least one metal selected from Sn, Sb, Fe, Co, Ni, Cu, Cr, Ti and Pb or at least one metal compound comprising these metals. Disadvantages are the expenditure required for solvent cleaning by comparison with gas phase cleavage, and the unavoidable loss of solvent.
EP 1 010 704 A2 discloses a one- or two-stage process for preparing alkoxysilanoalkylurethanes from alkoxysilanoalkylamines, urea and alcohol. The alkoxysilanoalkylurethanes can be cleaved thermally to give isocyanates containing alkoxysilane groups. The examples in the application cited disclose a process in which 3-aminopropyltriethoxysilane is first reacted with urea and ethanol, and ammonia formed and also unconverted ethanol, low boilers and compounds of higher molecular weight are removed. The carbamate obtained is then subjected to thermal cleavage in a cracking and rectification apparatus, wherein the ethanol released is drawn off at the top of the column, the 3-isocyanatopropyltriethoxysilane is withdrawn via the side draw, and a portion of the bottoms from the rectification unit comprising unconverted urethane is recycled into the carbamate preparation. However, the process has the disadvantage that the high boilers present in the cleavage bottoms material can lead to deposits in the apparatus and adversely affect the yield of the carbamate synthesis and hence the yield over the entire process. Moreover, recycling into the carbamate synthesis unnecessarily increases the complexity associated with distillative purification of the carbamate, which is also manifested in elevated capital and energy costs. Furthermore, the proportion of values present in the stream of matter from the cleavage bottoms is not optimally exploited in the sense of exhaustion of the yield potential.